Partial ionic ground state in the self-dimer mixed stack charge transfer crystal of TTF and TCNE

The vibrational spectra of the charge transfer complex of TTF with TCNE are explored. It is found that a degree of charge transfer of 0.5, namely that one electron has migrated from a dimer of TTF to that of TCNE molecules, characterizes the ground state of the complex. A Holstein-Hubbard model for a symmetric tetramer with two donor and two acceptor molecules allows the understanding of the vibronic bands observed in the infrared and Raman spectra and of the electronic charge transfer excitations.